HTLV-III/LAV is now recognized as the causative agent of AIDS and AIDS-related conditions. We have previously reported the molecular cloning and genetic analysis of a number of different HTLV-III/LAV isolates and have demonstrated that genomic variation is a characteristic property of this group of viruses. As a result, it has been speculated that genetic variation, especially within the envelope gene, may be intimately involved in the mechanisms by which HTLV-III/LAV evades host defenses thereby causing persistent and often fatal infection. It has also been postulated that envelope variation may result in altered tissue tropism and virulence of the virus, possibly explaining the variable clinical presentation of patients infected with HTLV-III/LAV. A limitation of all published studies to date, however, has been that the viral DNA characterized has been greatly expanded by in vitro viral cultivation prior to analysis. This has been necessary since only very small amounts of viral DNA are present in infected tissues at any one time. As a consequence, while much has been learned about the genetic organization and biology of laboratory isolates of HTLV-III/LAV, much less is known about the biology and genetics of HTLV-III/LAV as it exists in vivo. In this project, we propose to analyze and compare by nucleotide sequence analysis and by functional biologic analysis molecularly cloned isolates of HTLV-III/LAV from different body tissues with and without prior in vitro virus propagation. The specific aims of the project will be: 1. To determine if the virus isolated by in vitro cultivation of patient tissues accurately represents the viral forms (genotypes) present in vivo. 2. To determine if different patient tissues, e.g., blood lymphocytes, lymph node cells, bone marrow cells, and brain cells, harbor identical or different HTLV-III/LAV virus subtypes. 3. To molecularly clone the HTLV-III/LAV DNA genome directly from brain tissue of patients with AIDS encephalopathy and to determine if a neurotropic variant of HTLV-III/LAV possessing unique biological properties exists. 4. To characterize by restriction enzyme mapping and by nucleotide sequencing the extent, rate, and nature of viral genetic change that occurs during persistent viral infection in humans.